423: Throwing an Exception in C#

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Listing 423: Throwing an Exception
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using System; class ThrowingExceptions { static void Main() { try { ConsoleWriteLine("Begin executing"); ConsoleWriteLine("Throw exception"); throw new Exception("Arbitrary exception"); ConsoleWriteLine("End executing"); } catch (FormatException exception) { ConsoleWriteLine( "A FormateException was thrown"); } catch(Exception exception) { ConsoleWriteLine( "Unexpected error: {0}", exceptionMessage); } catch { ConsoleWriteLine("Unexpected error!"); } ConsoleWriteLine( "Shutting down"); }
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Basic Error Handling with Exceptions
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OUTPUT 415:
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Begin executing Throw exception Unexpected error: Arbitrary exception Shutting down
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As the arrows in Listing 423 depict, throwing an exception jumps execution from where the exception is thrown into the first catch block within the stack that is compatible with the thrown exception type In this case, the second catch block handles the exception and writes out an error message In Listing 423, there is no final block, so execution falls through to the SystemConsoleWriteLine() statement following the try/catch block In order to throw an exception, it is necessary to have an instance of an exception Listing 423 creates an instance using the keyword new followed by the data type of the exception Most exception types allow a message as part of throwing the exception so that when the exception occurs, the message can be retrieved Sometimes a catch block will trap an exception but be unable to handle it appropriately or fully In these circumstances, a catch block can rethrow the exception using the throw statement without specifying any exception, as shown in Listing 424
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Listing 424: Rethrowing an Exception
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catch(Exception exception) { ConsoleWriteLine( "Rethrowing unexpected error: exceptionMessage); throw; }
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Avoid Using Exception Handling to Deal with Expected Situations
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Developers should make an effort to avoid throwing exceptions for expected conditions or normal control flow For example, developers
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4: Methods and Parameters
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should not expect users to enter valid text when specifying their age5 Therefore, instead of relying on an exception to validate data entered by the user, developers should provide a means of checking the data before attempting the conversion (Better yet, you should prevent the user from entering invalid data in the first place) Exceptions are designed specifically for tracking exceptional, unexpected, and potentially fatal situations Using them for an unattended purpose such as expected situations will cause your code to be hard to read, understand, and maintain Additionally, (as with most languages) C# incurs a slight performance hit when throwing an exception taking microseconds compared to the nanoseconds most operations take This delay is generally not noticeable in human time except when the exception goes unhandled For example, when executing Listing 419 and entering an invalid age the exception is unhandled and there is a noticeable delay while the runtime searches the environment to see whether there is a debugger to load Fortunately, slow performance when a program is shutting down isn t generally a factor to be concerned with
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ADVANCED TOPIC Numeric Conversion with TryParse() One of the problems with the Parse() method is that the only way to determine whether the conversion will be successful is to attempt the cast and then catch the exception if it doesn t work Because throwing an exception is a relatively expensive operation, it is better to attempt the conversion without exception handling In the first release of C#, the only data type that enabled this was a double method called doubleTryParse() However, the CLI added this method to all numeric primitive types in the CLI 20 version It requires the use of the out keyword because the return from the TryParse() function is a bool rather than the converted value Here is a code listing that demonstrates the conversion using intTryParse()
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5 In general, developers should expect their users to perform unexpected actions, and therefore they should code defensively to handle stupid user tricks
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if (intTryParse(ageText, out age)) { SystemConsoleWriteLine( "Hi {0}! You are {1} months old", firstName, age * 12); } else { SystemConsoleWriteLine( "The age entered ,{0}, is not valid", ageText); }
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With the NET Framework 4, a TryParse() method was also added to enum types With the TryParse() method, it is no longer necessary to include a try/ catch block simply for the purpose of handling the string-to-numeric conversion
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